25 October 2010

Geological Frightfest: Tremors

Posted by Jessica Ball

Halloween is pretty much my favorite holiday – when else can I show up at school in costume? – and I’ve decided to resurrect the theme of geology in the movies for a Halloween series this week. To start us off, I’ll turn to that classic geologic monster movie: Tremors.


Tremors often comes up when lists of geology-themed movies are mentioned. Not only does it feature hungry subterranean critters, and a visiting seismologist, but at one point the heroes of the film are actually protected from the monsters by rocks. (Apparently tunneling through unconsolidated sediments is pretty easy, but granite boulders present a bit of a problem if you don’t have drill bits for teeth.) There’s also an appearance by Reba McEntire, for you music fans. 

The movie is supposedly set in the Nevada desert, but a little research shows that it was actually filmed in Lone Pine and Olancha, California, in Owens Valley just west of Death Valley National Park. The California State Geologic Map indicates that the area is mainly Mesozoic metavolcanic and plutonic rocks, with Quaternary sediments making up the valley fill. In case that wasn’t exciting enough, the Owens Valley Fault Zone cuts right through the area. (Garry Hayes of Geotripper has featured Owens Valley in a number of great posts – including one about some real tremors in the area!)

One thing that always bothered me about this movie was the “graboid” wormy things and how fast they were able to burrow through the ground. They don’t appear to have any means of locomotion other than wriggling and grabbing things with their mouths, but they somehow manage to chase down some very panicked humans. (According to this “Graboid 101” website, they have skin covered with “movement hooks” that help pull them through the ground and that their large jaws enable them to “dig through the land at high speed”, but I’m not buying it.) At a top speed of 15-20 mph (estimated here), these critters would far outstrip the top speed of an earthworm of the same scale. (To do some rough, totally gratuitous math: This paper estimates that a vigorous earthworm has a top speed of 10 mm/s, or 0.022 mph; assuming that a reasonable earthworm length is ~ 3 inches, scaling up to a 30-foot earthworm would still only get you a top speed of ~2.5 mph. Not even fast enough to catch a walking human!) Not to mention that this wouldn’t even account for variations in the consolidation of sediments or soil, having to detour around the odd buried boulder, etc.

So while this classic movie spawned three sequals and a 2003 TV series, you shouldn’t be worried about tremors of the graboid variety while visiting the Owens Valley – but watch out for earthquakes!